2,753 research outputs found
Catalytic RNA and synthesis of the peptide bond
We are studying whether the L-19 IVS ribozyme from Tetrahymena thermophila can catalyze the formation of the peptide bond when it is supplied with synthetic aminoacyl oligonucleotides. If this reaction works, it could give us some insight into the mechanism of peptide bond formation and the origin of coded protein synthesis. Two short oligoribonucleotides, CCCCC and a protected form of CCCCU were prepared; the former was made by the controlled hydrolysis of Poly(C), and the later by multistep chemical synthesis from the protected monomers. The homopentamer was then aminocylated using C-14 labelled Boc-protected glycine imidazolide. This aminoacylated oligo-nucleotide has now been shown to enter the active site of the L-19 IVS, and aminoacyl transfer, and peptide bond formation reactions are being sought. Our synthesis of CCCCU made us aware of the inadequacy of many of the 2'- hydroxyl protecting groups that are in use today and we therefore designed a new 2'- protecting group that is presently being tested
Quantum Hall induced currents and the magnetoresistance of a quantum point contact
We report an investigation of quantum Hall induced currents by simultaneous
measurements of their magnetic moment and their effect on the conductance of a
quantum point contact (QPC). Features in the magnetic moment and QPC resistance
are correlated at Landau-level filling factors nu=1, 2 and 4, which
demonstrates the common origin of the effects. Temperature and non-linear sweep
rate dependences are observed to be similar for the two effects. Furthermore,
features in the noise of the induced currents, caused by breakdown of the
quantum Hall effect, are observed to have clear correlations between the two
measurements. In contrast, there is a distinct difference in the way that the
induced currents decay with time when the sweeping field halts at integer
filling factor. We attribute this difference to the fact that, while both
effects are sensitive to the magnitude of the induced current, the QPC
resistance is also sensitive to the proximity of the current to the QPC
split-gate. Although it is clearly demonstrated that induced currents affect
the electrostatics of a QPC, the reverse effect, the QPC influencing the
induced current, was not observed
Colloquium: Physics of optical lattice clocks
Recently invented and demonstrated, optical lattice clocks hold great promise
for improving the precision of modern timekeeping. These clocks aim at the
10^-18 fractional accuracy, which translates into a clock that would neither
lose or gain a fraction of a second over an estimated age of the Universe. In
these clocks, millions of atoms are trapped and interrogated simultaneously,
dramatically improving clock stability. Here we discuss the principles of
operation of these clocks and, in particular, a novel concept of "magic"
trapping of atoms in optical lattices. We also highlight recently proposed
microwave lattice clocks and several applications that employ the optical
lattice clocks as a platform for precision measurements and quantum information
processing.Comment: 18 pages, 15 figure
Recommended from our members
High-voltage MOSFET bipolar square-wave generator
he construction of a high‐voltage (up to 1000 V) bipolar metal‐oxide‐semiconductor field‐effect transistor square‐wave generator is described. This generator is capable of producing both positive and negative going square waves with variable amplitude, repetition rate, and width. The circuit was designed for ferroelectrics research, however other applications are possible. The rise time of the prototype was 200 ns which was quite satisfactory for the present ferroelectric research project however the rise time can be decreased to 50 ns if necessary. The reader with a modest knowledge of electronics should be able to construct the circuit. Possible pitfalls and critical points are discussed
Comparing verbal media for alarm handling: Speech versus textual displays
The rise of computers in command and control domains has meant that control operations can be performed via desk-based visual display terminals. This trend has also produced the potential to display information to operators in a variety of formats. Of particular interest has been the use of text-based displays for alarm presentation. There are possible limitations to the use of text for alarm presentation, not least of which is the need for a dedicated alarms display screen (or, at least, a display page). Given the capability of computers to synthesize speech, it is possible that speech-based alarms could generate the same information as text-based displays without the need for dedicated screen space. In this paper an experimental comparison of speech-based and text-based displays for presentation of alarms is reported. The findings show that speech leads to longer response times than text displays, but that it has minimal effect on the efficacy of fault handling. The results are discussed within the alarm initiated activities framework and implications for alarm system design are outlined
Search strategies for supermassive stars in young clusters and application to nearby galaxies
Supermassive stars (SMS) with masses , formed
by runaway collisions in young, massive, and dense star clusters have been
invoked as a possible solution to the puzzles raised by the presence of
multiple stellar populations and peculiar abundance patterns observed in
globular clusters. However, such objects have not been observed so far. We
developed observational strategies to search for SMS hosted within young
massive clusters (thought to be the precursors of globular clusters, GCs),
which could be applicable in a relatively general fashion, using both
photometric and spectroscopic observations. We used theoretical predictions of
spectra of SMS and SMS-hosting clusters, together with predictions from
standard simple stellar populations to examine their impact on color-color
diagrams and on individual optical spectral lines (primarily Hydrogen emission
and absorption lines). As a first step, we apply our search strategies to a
sample of young star clusters (YSC) from two nearby galaxies with
multi-band observations from the HST and optical integral-field spectroscopy
obtained with MUSE on the Very Large Telescope. We focus on models for SMS with
large radii (corresponding to K), which predict strong
Balmer breaks, and construct proper color-color diagrams to select the
corresponding SMS-hosting cluster candidates. We show that their
spectrophotometric properties are similar to that of normal clusters with ages
of a few hundred Myr, which would, however, show signs of composite stellar
populations, in particular the presence of nebular lines (H and
others). Examining the photometry, overall SEDs, and the spectra of
clusters with strong Balmer breaks, we have found several objects with peculiar
SEDs, the presence of emission lines, or other peculiar signatures. [abridged]Comment: Accepted for publication in A&A, 15 pages, 12 figures, 2 tables,
(Abstract abridged
Risk Prediction Models for Colorectal Cancer: A Systematic Review.
Colorectal cancer is the second leading cause of cancer-related death in Europe and the United States. Survival is strongly related to stage at diagnosis and population-based screening reduces colorectal cancer incidence and mortality. Stratifying the population by risk offers the potential to improve the efficiency of screening. In this systematic review we searched Medline, EMBASE, and the Cochrane Library for primary research studies reporting or validating models to predict future risk of primary colorectal cancer for asymptomatic individuals. A total of 12,808 papers were identified from the literature search and nine through citation searching. Fifty-two risk models were included. Where reported (n = 37), half the models had acceptable-to-good discrimination (the area under the receiver operating characteristic curve, AUROC >0.7) in the derivation sample. Calibration was less commonly assessed (n = 21), but overall acceptable. In external validation studies, 10 models showed acceptable discrimination (AUROC 0.71-0.78). These include two with only three variables (age, gender, and BMI; age, gender, and family history of colorectal cancer). A small number of prediction models developed from case-control studies of genetic biomarkers also show some promise but require further external validation using population-based samples. Further research should focus on the feasibility and impact of incorporating such models into stratified screening programmes.J Usher-Smith is funded by a National Institute of Health Research (NIHR) Clinical Lectureship and F Walter by an NIHR Clinician Scientist award. J Emery is funded by an Australian National Health and Medical Research Council (NHMRC) Practitioner Fellowship. A Wong has an NHMRC Early Career Fellowship. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.This is the author accepted manuscript. The final version is available from American Association for Cancer Research via http://dx.doi.org/10.1158/1940-6207.CAPR-15-027
Magnetization Measurements of High-Mobility Two-Dimensional Electron Gases
De Haas–van Alphen (dHvA) oscillations are observed for Landau levels (LLs) with filling factors between 4 and 52, at temperatures in the range 50 mK to 1 K, in experiments on high-mobility GaAs/(Al, Ga)As heterojunctions. The oscillations become sawtooth-shaped at low filling factors, and theoretical fits to the data, assuming the two-dimensional electron gas to be a non-interacting Fermi system, show the shape of LLs to be close to a δ function. The small residual width (∼0.4 meV or less) fits equally well to either a Gaussian or a Lorentzian density-of-states model. In almost all cases, a constant background density of states has to be included to obtain a satisfactory fit. Weak odd-filling-factor dHvA peaks are detected at high fields, from which a g-factor enhancement of 15 can be inferred. Comparison of the scattering time derived from the fits before and after illumination, with the momentum relaxation time derived from transport, reveals a counterintuitive behavior in the bulk-modulation-doped sample
Magnetization measurements of high-mobility two-dimensional electron gases
De Haas-van Alphen (dHvA) oscillations are observed for Landau levels (LLs) with filling factors between 4 and 52, at temperatures in the range 50 mK to 1 K, in experiments on high mobility GaAs/(Al, Ga)As heterojunctions. The oscillations become sawtooth-shaped at low filling factors, and theoretical fits to the data, assuming the two-dimensional electron gas to be a non-interacting Fermi system, show the shape of LLs to be close to a ?-function. The small residual width (~0.4 meV or less) fits equally well to either a Gausian or a Lorentzian density of states model. In almost all cases, a constant background density of states has to be included to obtain a satisfactory fit. weak odd-filling-factor dHvA peaks are detected at high fields, from which a g-factor enhancement of 15 can be inferred. Comparison of the scattering time derived from the fits before and after illumination, with the momentum relaxation time derived from transport, reveals a counterintuitive behavior in the bulk-modulation-doped sample
Universal Prefactor of Activated Conductivity in the Quantum Hall Effect
The prefactor of the activated dissipative conductivity in a plateau range of
the quantum Hall effect is studied in the case of a long-range random
potential. It is shown that due to long time it takes for an electron to drift
along the perimeter of a large percolation cluster, phonons are able to
maintain quasi-equilibrium inside the cluster. The saddle points separating
such clusters may then be viewed as ballistic point contacts between electron
reservoirs with different electrochemical potentials. The prefactor is
universal and equal to 2 at an integer filling factor and to
2 at .Comment: 4 pages + 2 figures by reques
- …